Hi Kui,
well, I can see now that there are only binaries of the application.
But if you take closer look at the bootloader, you can see in linker file:
And then in main.c:

and
So, address 0x10003FFC contains pointer to function which you should call from application and address 0x2000EFFC contains status (return value) of that function which you are supposed to read after return.
In application, simply create a pointer to function:
void (*f_ptr)(void);
Load content of address 0x10003FFC to the pointer:
f_ptr = (void(*)(void))(*(unsigned int*)0x10003FFC);
And call the function:
f_ptr();
When you return from the function, simply read status from 0x2000EFFC using a pointer.
You do not need to save/restore register content because you are still using C and EABI.
Regards,
Lukas